In the production of rigid foam billet, boards or logs, a foamable thermoplastic resin such as styrene or polyurethane incorporating a blowing agent is extruded through a die orifice. As soon as the extrudate leaves the die orifice, it is at a much lower pressure, preferably a vacuum, and immediately begins to expand and move away from the die orifice. The extrudate will expand away from the die orifice increasing in size three dimensionally. If such expansion is not properly controlled and the extrudate properly drawn from the die orifice, particularly as the extrudate cools and begins to cure or set, undesirable dimensional variations may result such as corrugations as well as undesirable variations in density and cell orientation. Also, in the formation of boards, slabs and billets it is desirable to finish as nearly as possible at a uniform thickness the top and bottom planar surfaces of the board being formed. Accordingly, the finished product then need only be cut transversely with perhaps lateral edge trimming. It is of course important that the product be of uniform density, uniform thickness, and with minimum cell distortion. It is also important that the finished surface be free of distortions, surface blemishes or cracks.
Heretofore, a wide variety of roller, plate, or spreader bar arrangements have been employed to control and guide the formation of the expanding extrudate. Examples of such devices may be seen in the spreader bar of Winstead U.S. Pat. No. 3,632,266, the angularly related plates of Gilbert U.S. Pat. No. 3,431,163 and the idler roll arrangement seen in McCurdy et al U.S. Pat. No. 2,740,157.
In the case of idler rolls, such rolls can merely react to the force and movement of the extrudate moving therepast. Accordingly, it is preferred that such rolls should be driven to act in effect as traction rolls assisting in drawing the extrudate from the die orifice as it expands. In any die orifice, whether arcuate or linear, the extrudate will move past a straight roll at different speeds and in slightly different directions in view of lateral expansion. In extruding foamable resins such as styrenes, it is preferred to utilize a die having circular or parabolic, arcuate adjustable die lips such as seen in Phipps U.S. Pat. No. 3,871,812 or more preferably, as seen in the co-pending application of Arthur L. Phipps filed June 14, 1978, Ser. No. 914,580 entitled "Foam Extrusion Die Assembly". With such dies, the extrudate expands radially from the orifice defined by the die lips. Properly to assist the extrudate exiting the die orifice in this manner, the traction rollers assisting the movement of the extrudate and properly confining it to the desired shape should also, at least theoretically, be correspondingly curved. In this fashion, each portion of the driven roll will extend substantially normal to the movement of the extrudate therepast. Since the extrudate moves radially, the roller should be circular. To provide a circular roller, particularly one which can be rotated at a desired speed, and still one which can provide the desired pressure presents a complex problem.
Since any force exerted by an actuator acting on a roller assembly or a finishing surface above or below the extrudate path must compensate for the weight of the device being actuated in order to control precisely the pressures involved, applicants have devised a tare weight compensation control system for both the forming rolls and downstream surface finishing associated devices which permit both the rolls and the surface finisher to be floatingly supported and air balanced to compensate for its own weight so that the pressure exerted thereon toward the extrudate is the pressure exerted on the extrudate. In connection with the surface finishing device, it is supported at its center of gravity both to allow pivoting to compensate for slight further extrudate expansion and to permit a simplified system for air balancing and pressurizing the surface finisher.